In conventional tandem-type image-forming devices, it is difficult to manufacture the drum gears provided on the ends of the photosensitive drums and the drum drive gears disposed on the image-forming device side for driving the drum gears, without errors in shape or dimension. Such errors in the shape or dimension of the drum gear and drum drive gear cause the photosensitive drum, on which the drum gear is mounted, to rotate irregularly. When the image-forming device has a plurality of photosensitive drums, irregular rotations of the photosensitive drums may cause color registration problems in the image, resulting in a decline in image quality. Since the photosensitive drum is treated as a consumable that is discarded after use, the drum gear disposed on the photosensitive drum is normally formed of a resin material in order to minimize the manufacturing cost of the photosensitive drum. However, a resin gear is even more prone to errors in shape and dimension.
To resolve these problems, an image-forming device disclosed in Japanese Patent Application Publication No. HEI-11-30893 provides a worm wheel mounted on an end of each photosensitive drum. A mark is provided on part of the worm wheel. By detecting the mark with a sensor disposed near the rotating worm wheel, the image-forming device can control the rotational position, or phase, of the worm wheel.
However, the image-forming device described in Japanese Patent Application Publication No. HEI-11-30893 requires extra steps in the manufacturing process to form a mark on the worm wheel. This technology also requires extra space for the sensors and extra parts for supporting the sensors. Hence, the image-forming device has a complex structure in the engaging parts between the end of the photosensitive drum and the drum drive gear that drives the photosensitive drum, leading to a rise in the manufacturing cost of the image-forming device.
Another technology proposes to reduce this rise in cost by integrating the photosensitive drums for three colors and providing a photo interrupter and cam mechanism for the remaining photosensitive drum in order to align the phase of that photosensitive drum with the three integrated photosensitive drums.
However, this method requires that photosensitive drums for three colors be integrated. Therefore, when the image quality of a photosensitive drum for one color degrades and the drum must be replaced, the photosensitive drums for the other two colors whose image quality has not degraded must also be replaced, resulting in an unnecessary expense. Further, the drum drive gears mounted in the body of the image-forming device can be set to the same phase during assembly at the factory, and certainly the phase of the drum gears of the photosensitive drums that engage with the drum drive gears may also be set in phase at the factory. However, when the photosensitive drums are replaced with replacement parts, it is difficult to set the phase of the drum gear for the new photosensitive drum.